It can be appreciated that several trends presently exist in the electronics industry. Devices are continually getting smaller, faster and requiring less power, while simultaneously being able to support and perform a greater number of increasingly complex and sophisticated functions. One reason for these trends is an ever increasing demand for small, portable and multifunctional electronic devices. For example, cellular phones, personal computing devices, and personal sound systems are devices which are in great demand in the consumer market. These devices rely on one or more small batteries as a power source while providing increased computational speed and storage capacity to store and process data, such as digital audio, digital video, contact information, database data and the like.
Accordingly, there is a continuing trend in the semiconductor industry to manufacture integrated circuits (ICs) with higher device densities. To achieve such high densities, there have been and continue[[s]] to be efforts toward scaling down dimensions (e.g., at submicron levels) on semiconductor wafers. To accomplish such high densities, smaller feature sizes, smaller separations between features and layers, and/or more precise feature shapes are required, such as metal interconnects or leads, for example. The scaling-down of integrated circuit dimensions can facilitate faster circuit performance and/or switching speeds, and can lead to higher effective yield in IC fabrication processes by providing more circuits on a semiconductor die and/or more die per semiconductor wafer, for example.
One technique used in forming integrated circuits on semiconductor substrates is lithography, which broadly refers to processes for transferring one or more patterns between various media. In lithography, a light sensitive resist coating is formed over one or more layers to which a pattern is to be transferred (e.g., a semiconductor substrate). The resist is then patterned by exposing it to light, where the light (selectively) passes through an intervening lithography mask containing the pattern. The light causes the exposed or unexposed portions of the resist coating to become more or less soluble, depending on the type of resist used. A developer is then used to remove the more soluble areas leaving the patterned resist. The patterned resist can then serve as a mask for the underlying layers which can be selectively treated (e.g., etched) to transfer the pattern thereto.
In some types of lithography masks, such as phase shift masks, for example, the thickness of the masks is adjusted at select locations, such as via etching, for example, to produce desired behavior. In phase shift masks, for example, the thickness of the masks is adjusted at select locations to cause the phase of light passing through to be shifted by varying amounts (e.g., by diffraction). This creates positive and negative interference at desired locations to produce sharper, more defined features, edges, etc. In this manner, phase shifting can enhance the resolution of pattern transfers.
It can thus be appreciated that the accuracy with which integrated circuit patterns are formed on semiconductor substrates is, in large part, a function of the lithography mask used in transferring the patterns. The higher quality the lithography mask, the better the pattern transfers will be. Accordingly, it would be desirable to accurately and precisely produce a lithography mask which can be used to transfer patterns of reduced dimensions onto a semiconductor substrate to facilitate device scaling.
A lithography mask is generally constructed by depositing a substantially opaque layer of material on a surface of a transmissive substrate, where the substrate is substantially transparent to the light used to effect lithographic pattern transfers. Portions of the opaque layer are removed to form the pattern to be transferred to the wafer. With regard to phase shift masks, for example, a thickness of the substrate is reduced at various locations to yield desired phase shifts. Reducing the thickness of the light transmitting substrate may be accomplished by etching or other suitable processes. For example, the substrate may be etched at select locations until a desired thickness remains to yield desired phase shifting.
However, conventional methods for fabricating lithography masks have several drawbacks. For example, aside from the duration of an etching process and/or the chemistry of a homogeneous blanket film of the etching process, the precision of resulting parameters from lithography mask fabrication etch processes is difficult to control. This can, for example, result in imprecise thicknesses of the light transmitting substrate. As a result, an actual phase shift can vary from a desired phase shift due to a deviation between an obtained lithography mask substrate thickness and a desired lithography mask substrate thickness. Further, dry etching techniques are generally isotropic—meaning that they remove equal amounts of material in both horizontal and vertical directions. This makes it difficult to form a deep yet narrow feature in a lithography mask, as may be desired to advance device scaling. In addition, the width of such a feature becomes proportional to the depth required to achieve a certain phase shift. This reduces the ability to control critical dimensions independently of phase shift.